Control of Perovskite Crystal Growth by Methylammonium Lead Chloride Templating

Article abstract of DOI:10.1002/asia.201501379

State-of-the-art solar cells based on methylammonium lead iodide (MAPbI₃) now reach efficiencies over 20 %. This fast improvement was possible with intensive research in perovskite processing. In particular, chloride-based precursors are known to have a positive influence on the crystallization of the perovskite. Here, we used a combination of in-situ X-ray diffraction and charge-transport measurements to understand the influence of chloride during perovskite crystallization in planar heterojunction solar cells. We show that MAPbCl₃ crystallizes directly after the deposition of the starting solution and acts as a template for the formation of MAPbI₃. Additionally, we show that the charge-carrier mobility doubles by extending the time for the template formation. Our results give a deeper understanding of the influence of chloride in the synthesis of MAPbI₃ and illustrate the importance of carefully controlling crystallization for reproducible, high-efficiency solar cells.

Full text of DOI:10.1002/asia.201501379

DOI: 10.1002/asia.201501379
Communication
Hybrid Perovskites
Control of Perovskite Crystal Growth by Methylammonium Lead
Chloride Templating
Andreas Binek+, Irene Grill+, Niklas Huber+, Kristina Peters, Alexander G. Hufnagel,
Matthias Handloser, Pablo Docampo, Achim Hartschuh, and Thomas Bein*[a]
the resulting device performance.[14–16] Hence, understanding
Abstract: State-of-the-art solar cells based on methylam-
the crystallization mechanism of the active layer is a crucial
monium lead iodide (MAPbI3) now reach efficiencies over
factor. In grain boundaries and defects, traps for the photoex-
20%. This fast improvement was possible with intensive
cited species are located and recombination can occur.[17] Thus,
research in perovskite processing. In particular, chloride-
a good strategy to maximize device efficiency is the reduction
based precursors are known to have a positive influence
of the number of grain boundaries in the perovskite layer by
on the crystallization of the perovskite. Here, we used
tuning the deposition technique.[18] A common approach to
a combination of in-situ X-ray diffraction and charge-trans-
improve the performance of perovskite solar cells is the use of
port measurements to understand the influence of chlo-
a chloride-based precursor in the casting solution. In particular,
ride during perovskite crystallization in planar heterojunc-
lead chloride (PbCl2) and methylammonium iodide (MAI) mix-
tion solar cells. We show that MAPbCl3 crystallizes directly
tures lead to highly efficient devices when employed in planar
after the deposition of the starting solution and acts as
heterojunction solar cells.[19] This enhancement is generally at-
a template for the formation of MAPbI3. Additionally, we
tributed to the formation of bigger crystal domains of the per-
show that the charge-carrier mobility doubles by extend-
ovskite compared to other precursors and the related reduc-
ing the time for the template formation. Our results give
tion of grain boundaries and defects in the bulk material.[16]
a deeper understanding of the influence of chloride in the
However, the role of chloride during the crystallization process
synthesis of MAPbI3 and illustrate the importance of care-
is still unclear, with many groups reporting no or only small
fully controlling crystallization for reproducible, high-effi-
amounts of chloride in the final structure.[20–23] Thus, under-
ciency solar cells.
standing the crystallization mechanism and, therefore, gaining
additional handles to tune the crystal morphology of these sys-
tems can open up new strategies to maximize the device per-
With rising global energy demands and the decline of fossil
fuels reserves, there is a great need to develop renewable
energy resources. Lately, solar cells based on organic-inorganic
trihalide perovskites, for example, (CH3NH3)PbI3, have emerged
as a highly efficient and inexpensive photovoltaic technolo-
gy.[1–4] Through optimization of the fabrication processes,[5–7]
the annealing process,[8–9] and the interfaces,[10] perovskite solar
cells have already exceeded 20% power conversion efficiency
(PCE).[11] Recently, several novel crystallization methods have
been exploited to fabricate perovskite photovoltaic devices
with high and reproducible performance.[12–13] In particular, pre-
vious studies show that control of the macroscopic morpholo-
gy and the crystalline domain size has a strong influence on
formance. Here, we investigate the crystallization of the perov-
skite immediately after deposition in order to understand and
control the crystallization kinetics of the system. Our in-situ X-
ray diffraction measurements show that initially MAPbCl3 crys-
tallizes on the substrates to be fully converted into MAPbI3
after a certain time under heating. Furthermore, we show the
positive influence of the slow evaporation of the solvent on
the crystal size, morphology, and also on the charge-carrier
mobility in the perovskite layer. Our results help to understand
the influence of chloride during the crystallization process of
the perovskite and the origin of the improved performance of
the system.
The MAPbI3 samples were prepared according to the synthe-
sis route of Eperon et al.[8] In short, a mixture of lead chloride
(PbCl2) and methylammonium iodide (MAI) was dissolved at
a ratio of 1:3, and was then spin-coated on a TiO2-covered fluo-
rine-doped tin oxide (FTO) substrate. Afterwards, the sample
was kept at room temperature (RT) for different times to allow
for the slow evaporation of the residual solvent, and then
heated to different temperatures to complete the conversion
to the MAPbI3 crystal phase. With this procedure we enabled
slow formation of the perovskite, leading to uniform crystalli-
tes. In order to examine the influence of different evaporation
times of the solvent at RT on the perovskite morphology,
[a] A. Binek,+ I. Grill,+ N. Huber,+ K. Peters, A. G. Hufnagel, Dr. M. Handloser,
Dr. P. Docampo, Prof. A. Hartschuh, Prof. T. Bein
Department of Chemistry and Center for NanoScience (CeNS)
University of Munich (LMU)
Butenandtstr. 5–13 (Haus E), 81377 Munich (Germany)
E-mail: bein@lmu.de
[+] These authors contributed equally to this work.
Supporting information for this article can be found under http://
dx.doi.org/10.1002/asia.201501379.
This manuscript is part of a special issue on energy conversion and storage.
Click here to see the Table of Contents of the special issue.
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cross-sections were prepared (Figure 1). Short exposure times
between 1 min and 20 min resulted in crystallites that are not
uniform in size and shape, and a large number of grain boun-
daries are visible. In contrast, the SEM images of samples with
a longer crystallization time show larger and more uniform
crystallites, and consequently a reduction of grain boundary
density.
therefore ruling out MAPbI3ÀxClx mixed phases. The broad re-
flection between 11.2 and 12.682q is likely the result of the for-
(2Àn)
mation of PbIn
complexes, which arise as a result of em-
ploying non-stoichiometric perovskite mixtures with an excess
of the organic cation.[24] The difference between both struc-
tures is not only the halide but also the arrangement of the
PbX6 octahedra. Heating to 908C leads to a reversal of reflec-
tion intensity of both perovskite structures, and thus conver-
sion from one structure to the other. Thereby, the intensity of
MAPbCl3 is quickly decreased while the reflection of MAPbI3
gains in intensity. Furthermore, the second reflection of the
iodide perovskite at around 2882q also increases during the
heat treatment. In the following two temperature steps (Fig-
ure 2c), the intensity of MAPbCl3 reflections is further de-
creased and the orientation of the MAPbI3 crystals is enhanced
as the number of reflections is reduced, and only reflections
arising from c-axis orientation remain. After cooling down to
RT, only two reflections are observed, indexed as (001) and
(002) from MAPbI3, indicating that the crystals exhibit a pre-
ferred orientation along the c-axis and therefore parallel to the
substrate. The same orientation is also observed in samples
having been exposed to different evaporation times at RT, as
shown in Figure S1 in the Supporting Information. From a crys-
tallographic point of view the samples are very similar, howev-
er, the cross-sections show different morphologies.
To substantiate the uniformity of the MAPbI3 crystals,
smooth cross sections showing high contrast were also pre-
pared with a focused ion beam (FIB). In the FIB cross-sections
(Figure 1b) the noticeable difference between fast and slow
evaporation of the solvent is even more visible. The 5 min
sample exhibits a rough surface due to non-uniform crystal for-
mation. In comparison, the 40 min sample shows a regular
brick-like morphology of the perovskite on the substrate. In
order to further understand the crystallization and the influ-
ence of the different temperature treatments during the syn-
thesis, in-situ X-Ray diffraction measurements (XRD) were per-
formed under a nitrogen atmosphere with freshly spin-coated
samples. Additionally, we investigated the effect of heating
steps on the perovskite crystal structure; such steps are often
used to achieve highly efficient devices for a range of exposure
times (Figure 2). In the first XRD pattern, after 5 min at RT, an
intense reflection at around 1682q is observed. Over time, an
additional reflection at 1482q appears and slowly increases at
RT. The first reflection at higher angles (1682q) is attributed to
the (200) plane of the MAPbCl3 perovskite structure, while the
second reflection (1482q) is attributed to the (002) plane of
MAPbI3 (Figure 2d); no shift is present in either reflection,
Crystallization of a material from a solution can occur by
cooling down the solution or through evaporation of the sol-
vent.[25] In the case of the one-step synthesis route for MAPbI3,
supersaturation is induced by evaporating the solvent. This is
Figure 1. SEM images of cross-sections of samples with different evaporation times at RT (a) and SEM images of FIB cross-sections of a 5 min and a 40 min
sample, respectively (b).
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Figure 2. In-situ X-ray diffraction measurements with samples prepared according to the PbCl2 route. XRD patterns for the RT step from 5 min to 60 min (a),
908C step from 2 min to 120 min (b), 1008C/1308C at different times and after cooling down (c). Theoretical patterns of MAPbI3 and MAPbCl3 compared with
XRD pattern of the sample after 20 min at RT (d).
achieved through spinning of the substrate, which then leads
to the formation of the nuclei. Afterwards, the crystallites grow
due to the heating of the substrate. The fact that chloride,
which improves the crystal quality of the perovskite,[26] is not
incorporated into the structure of MAPbI3, indicates that its
effect must take place during the crystallization process.
for MAPbI3. Additionally, some crystallites of MAPbI3 emerge
during the RT treatment as observed in the in-situ XRD. These
can then act as seeds for the formation of the MAPbI3 layer
while heating up the system. Over time, the solvent as well as
the excess of MA and chloride are evaporated, and highly ori-
ented MAPbI3 crystals are formed on the surface of the sub-
strate with the orientation induced by the MAPbCl3 template.
With this crystallization mechanism, the morphological differ-
ences in the SEM cross-sections can be explained with the
time required by the MAPbCl3 template to form on the surface.
A short time of 5 min at RT appears to be insufficient to create
an oriented and fully formed MAPbCl3 template. Therefore, in
this case, the deposition of MAPbI3 during the heat treatment
leads to less uniform and smaller crystals. The difference be-
tween samples treated for 5 min and 40 min at room tempera-
ture was further investigated by incorporating the resulting
The crystallization order of the two perovskite structures can
be explained with the Ostwald–Volmer rule, which states that
the system with the lower density crystallizes first.[27] Here, the
[28]
density of MAPbCl3
was estimated as 1.576 gcmÀ3, while
a value of 4.119 gcmÀ3 was obtained for the MAPbI3 com-
pound, and therefore the chloride-containing material is ex-
pected to crystallize first, as observed experimentally. We show
a schematic of the proposed mechanism in Figure 3. Here, the
sample is left at RT for the slow evaporation of the solvent,
where MAPbCl3 forms on the surface and acts as a template
[29]
Figure 3. Schematic mechanism of the crystallization of the MAPbI3 perovskite based on the PbCl2 synthesis route in accordance with the Ostwald–Volmer
rule.
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perovskite layers in a planar heterojunction photovoltaic
device architecture with TiO2 and spiro-OMeTAD serving as the
charge extraction contacts.[30] The current/voltage curves and
the efficiency distributions of the devices are displayed in Fig-
ure 4a and 4b, respectively. We show that the film treated for
40 min at RT exhibits enhanced PCE values in comparison to
the film treated for 5 min at RT. We attribute this difference to
the higher crystallinity and more uniform size distribution of
crystals within the film during the MAPbCl3 template-assisted
growth process, as illustrated by our in-situ XRD measurements
depicted in Figure 2a–d. In the SEM top-view images (Fig-
ure S2, Supporting Information) of both films we observe that
the domain sizes of the perovskite crystals are increased while
the surface density of pinholes is decreased for the film treated
for 40 min at RT as compared to the film treated for 5 min,
leading to an enhanced PCE and reduced hysteresis of the
device (Figure S3, Supporting Information).[16] The trend ob-
served for the best device J–V curves, as presented in Figure 4,
also holds for averaged devices as shown in Figure S4. Addi-
tionally, the greater number of pinholes present in the film
treated for 5 min at RT also results in a small open circuit volt-
age loss due to higher recombination losses, similar to previ-
ous results reported in the literature.[31]
ToF is a well-established method to extract the mobility of
charge carriers in semiconductors and solar cells. Based on the
generation of charge carriers by short laser pulses and subse-
quent drifting due to an applied bias, the resulting time-re-
solved photocurrent is used for the determination of the corre-
sponding mobility.[32] A representative ToF transient is depicted
in Figure 4c. From this transient, the transit time ttr is obtained.
For a detailed description of the fitting procedure and the
measurement setup used, including the transit time as a func-
tion of applied bias, see Figures S5–S7 in the Supporting Infor-
mation. The time resolution of the system is ~2 ns allowing for
the extraction of thin film mobilities after weak excitation fol-
lowing Equation (1):
m ¼ d2=U Á ttr
ð1Þ
Here, d is the active layer thickness and U the applied bias volt-
age. In the current device structure, we predominantly mea-
sure hole transport, as most charges are generated in close
proximity to the electron collection layer (TiO2), while the holes
must travel throughout the whole thickness of the film. We
note that the resulting ToF transient characteristics in a thin
film device fabricated for supporting electron transport, for in-
stance, by using PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-
polystyrene sulfonate) and PCBM (phenyl-C61-butyric acid
methyl ester), would lead to similar results, since electron and
In order to investigate the relationship between crystal qual-
ity and transport dynamics in our solar cells, we carried out
time-of-flight (ToF) studies for the devices fabricated from per-
ovskite films treated for 5 and 40 min at room temperature.
Figure 4. J–V characteristics of MAPbI3-based solar cell devices prepared with 5 min and 40 min evaporation time at RT (a) and their distribution of efficiencies
from 200 solar cells (b). Panel c shows a representative ToF transient. The inset is a double logarithmic zoom-in of the region used to fit the data to obtain
the transit time ttr (see text). The respective mobilities obtained by ToF measurements of both sample types are depicted in panel d. Here it is observed that
the incorporation of the material treated for 40 min at RT in the solar cell leads to an increase in charge-carrier mobility by a factor of 2.
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hole mobility in MAPI thin films have been shown to be simi-
lar.[33] Further, the penetration depth of light in MAPI thin films
at 550 nm is 0.66 mm, as measured by Park.[34]
Experimental Section
Preparation of the precursor: Methylammonium iodide was pre-
pared by adapting a recipe published earlier.[38] In short, 24 mL of
methylamine solution (33% in ethanol, Sigma–Aldrich) was diluted
with 100 mL of absolute ethanol in a 250 mL round-bottom flask.
To this solution, 10 mL of hydroiodic acid (33 wt%) was added
under constant stirring. After a reaction time of one hour at room
temperature, the solvents were removed by rotary evaporation.
The obtained white solid was washed with dry diethyl ether and fi-
nally recrystallized from ethanol.
The results extracted from ToF data can be found in Fig-
ure 4d. We note that the experimental error is mainly caused
by the determination of the layer thickness. The slow evapora-
tion of the solvent and the associated formation of uniform
and big crystals of the perovskite for the 40 min at RT sample
can be connected to the enhancement of the mobility of the
charge carriers within the photoactive layer, as observed in the
ToF measurements. The higher mobility in the 40 min at RT
sample is also observable in the J–V characteristic since the
series resistance of the solar cell is lowered, which leads to
a higher fill factor (FF). The series resistance was estimated by
fitting the ohmic regime of the J–V curve and yields
12.5 Wcm2 for the 5 min and 8.2 Wcm2 for the 40 min device.
With this increased conductivity of the sample, the charge ex-
traction by the selective contacts is enhanced, resulting in
higher device efficiency. The results presented here show mo-
bility values two orders of magnitude higher than previous ToF
studies on perovskite solar cells.[35] We note that the morpholo-
gy of the perovskite plays an important role for the mobility,
as demonstrated here, and hence we expect that the much
smaller crystals and thus higher grain boundary density pres-
ent in the previous study account for the discrepancy. On the
other hand, values obtained from THz (8.1 cm2 VÀ1 sÀ1)[36] and
microwave (6.2 cm2 VÀ1 sÀ1)[37] conductivity measurements on
the active perovskite layer only are up to three orders of mag-
nitude higher than the present results that have been ob-
tained for complete devices. We attribute these differences to
the different transport processes and probing dimensions asso-
ciated with THz and microwave measurements, in comparison
with the complete through-layer transport probed with ToF
methods. Moreover, as the ToF measurement probes the entire
device, there could also be a minor influence of the transport
layer (spiro-OMeTAD) and the relevant interfaces on the appar-
ent mobility of the material.
Solar cell preparation: Fluorine-doped tin oxide (FTO)-coated glass
sheets (7 WsqÀ1, Pilkington, USA) were patterned by etching with
zinc powder and 3 M HCl. They were subsequently cleaned with
a 2% Hellmanex solution and rinsed with deionized water, ethanol,
and acetone. Directly before applying the blocking layer, remaining
organic residues were removed by an oxygen plasma treatment
for 5 min. The dense TiO2 layer was prepared from a sol-gel precur-
sor solution by spin-coating onto the substrates and calcining at
5008C in air.[39] For the sol-gel solution a 27.2 mM (70 mL) solution
of HCl in 2-propanol (5 mL) was added dropwise to a vigorously
stirred 0.43 mM (735 mL) solution of titanium isopropoxide
(99.999%, Sigma–Aldrich) in 2-propanol (5 mL). The solution usual-
ly stayed clear during the addition and was discarded otherwise.
The precursor solution for the synthesis of MAPbI3 was prepared
by dissolving 1.685 g of MAI in 4 mL dry N,N-dimethylformamide
(DMF, 99.8%, Sigma–Aldrich). This solution was then added to
973 mg of PbCl2 (98%, Sigma–Aldrich) and heated to 1008C in
order to fully dissolve the lead precursor. Subsequently, 100 mL of
this solution was spin-coated onto the TiO2-covered substrates at
1000 rpm for 45 s. After varying the resting times at RT, the sam-
ples were placed on a hotplate at 908C for 2 h. Afterwards, two ad-
ditional heating steps were performed, first 10 min at 1008C and
then 5 min at 1308C. Next, the films were covered with a layer of
Spiro-OMeTAD (Borun Chemicals, 99.5% purity). Typically, 100 mg
of Spiro-OMeTAD were dissolved in 1 mL chlorobenzene (99.8%,
Sigma–Aldrich). The solution was filtered and mixed with 10 mL 4-
tert-butylpyridine (tBP, 96%, Sigma–Aldrich) and 30 mL of
a
170 mgmLÀ1 bis(trifluoromethane)sulfonamide lithium salt
(LiTFSI, 99.95%, Sigma–Aldrich) solution in acetonitrile. This solu-
tion was spin-coated dynamically at 1500 rpm for 45 s. In a second
step the sample rotation was accelerated to 2000 rpm for 5 s to
allow the solvent to dry completely. Before depositing the gold
electrodes by evaporation, Spiro-OMeTAD was left to oxidize in air
overnight at room temperature and <20%rel humidity.
In summary, we have studied the crystallization of MAPbI3
based on a one-step approach with a chloride-based precursor.
Based on in-situ XRD measurements, we propose a crystalliza-
tion mechanism for the synthesis procedure. Thereby, MAPbCl3
is assembled on the substrate during the slow evaporation of
the solvent, and over time some MAPbI3 crystals are also
formed. While heating up the substrate, MAPbI3 grows at the
expense of MAPbCl3, which leads to crystals oriented parallel
to the substrate. Furthermore, we show that a slow evapora-
tion of the solvent during the formation of the MAPbCl3 tem-
plate influences the morphology, size, and uniformity of the re-
sulting MAPbI3 crystals. Advanced electro-optical characteriza-
tion by time-of-flight studies in this work showed that the
charge-carrier mobility is doubled for devices based on MAPbI3
that were fabricated with more controlled evaporation of the
solvent at RT. This indicates that slow evaporation of the sol-
vent before the heat treatment benefits the solar cell efficiency
through enhanced conductivity and a corresponding increased
device performance.
Acknowledgements
The authors wish to thank Steffen Schmidt from the Depart-
ment of Chemistry at the University of Munich (LMU) for the
focused ion beam cross-sections. We acknowledge funding
from the Bavarian State Ministry of the Environment and Con-
sumer Protection, the Bavarian network “Solar Technologies Go
Hybrid”, and the DFG Excellence Cluster Nanosystems Initiative
Munich (NIM). We gratefully acknowledge support from the Eu-
ropean Union through the award of a Marie Curie Intra-Euro-
pean Fellowship. The authors acknowledge funding from the
German Federal Ministry of Education and Research (BMBF)
under the agreement number 01162525/1.
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Keywords: crystallization · hybrid perovskite · in-situ X-ray
diffraction · methylammonium lead iodide · time-of-flight
charge-carrier mobility
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